The anionic polymerization of non-polar monomers such as styrene and dienes are well known in the prior act (U.S. Pat. Nos. 2,351,924; 4,414,372; Brit. Patent No. A-0 145 263; Macromolecules 1984, 17, 1415-1417). The feasibility of preparing well-defined homo and block copolymers with terminal-functional groups has been demonstrated using this technique. However, the extension of this method to polar monomers such as alkyl(meth)acrylates is complicated and requires specific experimental conditions that minimizes the occurence of secondary reactions arising due to the nucleophilic attack of carbonyl group and acidic-hydrogen atom of the acrylates (Polymers, 3, 175,1962; J. Polym. Sci.
Poly. Chem., 12, 2295,1974; and Polym. J., 4, 271, 1973).
The secondary side reactions in anionic polymerization of alkyl(meth)acrylate are basically due to the solvation of metal ions by carbonyl group, present in both the monomer and the propagating species.
To avoid such side reactions, many approaches are documented in the prior art. The external solvation of the metal ion by a polar solvent at low temperature helps in controlling ion-pair aggregation and minimizes the secondary reactions. Thus, for successful living anionic polymerization of alkyl(meth)acrylate one requires the use of a sterically bulky anion in polar solvents such as tetrahydrofuran (THF) at low temperature, typically less than -70.degree. C. Atempts have been made to alter the reactivity of the propagating species by adding adjuvants such as solvating agents (J. Polym. Sci. Part A, Poly. Chem., 28, 1033, 1990), alkali metal alkoxides (Makromol. Chem., 191, 1657,1990), alkali halides (Luxemborg patent 85627, Nov. 5, 1984; U.S. Pat. No. 4,767,824,1988), aluminum alkyls (Polym. Bull., 20(6),505, 1988). Other new initiator systems have also been described for the living anionic polymerization of alkyl(meth)acrylates. These are, aluminum porphyrin(Macromolecules, 20, 3076,1987), metal free anionic initiators (Polym. Prep. 32(1), 296, 1991 and Macromolecules, 24, 1697, 1991) and other metal catalysts (J. Am. Chem. Soc., 114,5460 and 4908, 1992).
Highly stabilized anionic initiators bearing tetra-n-butyl ammonium cations have been claimed as good initiating systems for alkyl(meth)acrylates at 25.degree. C. and above (Angew. Chem. Int. Ed. Engl. 27,1373, 1988; and U.S. Pat. No. 5,194,537, 1993). The process of nucleophile assisted group transfer polymerization of silyl ketene acetals is also useful for the living polymerization of alkyl(meth)acrylates and other a,B,-unsaturated carbonyl monomers. Substantial progress has been achieved recently in the controlled polymerization of alkyl(meth)acrylates by group transfer polymerization method (J. Am. Chem. Soc., 105, 5706, 1983; New Methods of Polymer Synthesis, J. R. Ebdon; Ed, Blackie, pp 22-75, 1991).
However, one of the drawbacks of the group transfer polymerization method, is the difficulty in synthesizing block copolymer of alkyl(meth)acrylates with monomers such as styrene, dienes and heterocyclic monomers. Resonance stabilized metal free carbanion initiators are also limited to only alkyl(meth)acrylates and does not allow sequential block copolymerization with styrene and dienes.
It is well known that use of specific adjuvants in the anionic polymerization of alkyl(meth)acrylates modify the reactivity of propagating chain end and thus help to minimize undesirable side reactions during propagation. For example, tert-alkoxides as adjuvants, stabilizes the propagating chain end and is used to suppress the intramolecular cyclization (auto termination) reaction (Polym. Prep., 29(2), 29, 1988; Makromol. Chem., 193,101,1992, and J. Makromol. Chem., 194, 841, 1993).
The use of an alkali halide such as LiCl also has a beneficial effect and plays a major role in influencing the dynamic equilibrium of aggregated and non-aggregated ion-pairs during polymerization. In presence of LiCl, anionic polymerization of methyl methacrylate and t-butylacrylate proceed satisfactorily, giving good control on molecular weight and polydispersity. However, such a control in presence of LiCl can only be achieved in a polar solvent such as tetrahydrofuran at -78.degree. C. (Macromolecules, 24, 4997, 991; 20, 1442, 1987; and 23, 2618, 1990). Using conventional anionic initiators such as 1,1'-diphenylhexyl lithium, alpha-methyl styryl lithium, diphenyl methyl lithium and alkyl alpha-lithioisobutyrate and LiCl, no control on polymerization of alkyl(meth)acrylate can be achieved in toluene or toluene-tetrahydrofuran mixtures (9:1 v/v).
Anionic polymerization of methyl methacrylate in toluene-tetrahydrofuran 9:1 (v/v) solvent at -78.degree. C. is complicated and often leads to gel formation due to uncontrolled propagation reaction. This situation can be improved by addition of LiCl but only broad MWD (1.5) poly methyl methacrylate can be obtained at -78.degree. C. (Macromolecules 23, 2618-2622, 1990).